Question

How syringe draws blood from our body?

Answer 1

When a doctor inserts the needle into the vein, then slides evacuated tubes into the tube holder, where the tube's stopper is pierced by the back end of the needle, a vacuum is created in the syringe.

The vacuum in the tube then automatically draws the needed blood directly from the patient's vein.

Pressure difference created between syringe and blood is responsible for the blood movement
Hope it helps